Align erasure of Array[Nothing] and Array[Null] with Scala 2 (#22517)

Closes #22515

Author: hamzaremmal

Diff for: compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -326,7 +326,7 @@ object TypeErasure {
         val sym = t.symbol
         // Only a few classes have both primitives and references as subclasses.
         if (sym eq defn.AnyClass) || (sym eq defn.AnyValClass) || (sym eq defn.MatchableClass) || (sym eq defn.SingletonClass)
-           || isScala2 && !(t.derivesFrom(defn.ObjectClass) || t.isNullType) then
+           || isScala2 && !(t.derivesFrom(defn.ObjectClass) || t.isNullType | t.isNothingType) then
           NoSymbol
         // We only need to check for primitives because derived value classes in arrays are always boxed.
         else if sym.isPrimitiveValueClass then
@@ -596,8 +596,8 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
    *   will be returned.
    *
    *  In all other situations, |T| will be computed as follow:
-   *   - For a refined type scala.Array+[T]:
-   *      - if T is Nothing or Null, []Object
+   *   - For a refined type scala.Array[T]:
+   *      - {Scala 2} if T is Nothing or Null, []Object
    *      - otherwise, if T <: Object, []|T|
    *      - otherwise, if T is a type parameter coming from Java, []Object
    *      - otherwise, Object
@@ -781,12 +781,14 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
 
   private def eraseArray(tp: Type)(using Context) = {
     val defn.ArrayOf(elemtp) = tp: @unchecked
-    if (isGenericArrayElement(elemtp, isScala2 = sourceLanguage.isScala2)) defn.ObjectType
-    else
-      try
-        val eElem = erasureFn(sourceLanguage, semiEraseVCs = false, isConstructor, isSymbol, inSigName)(elemtp)
-        if eElem.isInstanceOf[WildcardType] then WildcardType
-        else JavaArrayType(eElem)
+    if isGenericArrayElement(elemtp, isScala2 = sourceLanguage.isScala2) then 
+      defn.ObjectType
+    else if sourceLanguage.isScala2 && (elemtp.hiBound.isNullType || elemtp.hiBound.isNothingType) then
+      JavaArrayType(defn.ObjectType)
+    else 
+      try erasureFn(sourceLanguage, semiEraseVCs = false, isConstructor, isSymbol, inSigName)(elemtp) match
+        case _: WildcardType => WildcardType
+        case elem => JavaArrayType(elem)
       catch case ex: Throwable =>
         handleRecursive("erase array type", tp.show, ex)
   }

Diff for: sbt-test/scala2-compat/erasure/build.sbt

@@ -1,3 +1,5 @@
+ThisBuild / fork := true
+
 lazy val scala2Lib = project.in(file("scala2Lib"))
   .settings(
     scalaVersion := sys.props("plugin.scala2Version")

Diff for: sbt-test/scala2-compat/erasure/dottyApp/Api.scala

@@ -195,4 +195,10 @@ class Z {
   def objectARRAY_88(x: Array[Any]): Unit = {}
   def objectARRAY_89(x: Array[AnyRef]): Unit = {}
   def objectARRAY_90(x: Array[AnyVal]): Unit = {}
+
+  def nothing$ARRAY_91(x: Array[Nothing]): Unit = {}
+  def null$ARRAY_92(x: Array[Null]): Unit = {}
+  def nothing$ARRAY_93(x: Array[_ <: Nothing]): Unit = {}
+  def null$ARRAY_94(x: Array[_ <: Null]): Unit = {}
+
 }

Diff for: sbt-test/scala2-compat/erasure/dottyApp/Main.scala

@@ -53,10 +53,10 @@ object Main {
     z.c_40(dummy)
     z.c_41(dummy)
     z.c_42(dummy)
-    z.b_43(dummy)
+    //z.b_43(dummy)
     z.c_44(dummy)
     z.c_45(dummy)
-    z.b_46(dummy)
+    //z.b_46(dummy)
     z.c_47(dummy)
     // z.a_48(dummy)
     // z.c_49(dummy)
@@ -101,6 +101,10 @@ object Main {
     z.objectARRAY_88(dummy)
     z.objectARRAY_89(dummy)
     z.objectARRAY_90(dummy)
+    z.objectARRAY_91(dummy)
+    z.objectARRAY_92(dummy)
+    z.objectARRAY_93(dummy)
+    z.objectARRAY_94(dummy)
 
     val methods = classOf[scala2Lib.Z].getDeclaredMethods.toList ++ classOf[dottyApp.Z].getDeclaredMethods.toList
     methods.foreach { m =>

Diff for: sbt-test/scala2-compat/erasure/scala2Lib/Api.scala

@@ -186,4 +186,10 @@ class Z {
   def objectARRAY_88(x: Array[Any]): Unit = {}
   def objectARRAY_89(x: Array[AnyRef]): Unit = {}
   def objectARRAY_90(x: Array[AnyVal]): Unit = {}
+
+  def objectARRAY_91(x: Array[Nothing]): Unit = {}
+  def objectARRAY_92(x: Array[Null]): Unit = {}
+  def objectARRAY_93(x: Array[_ <: Nothing]): Unit = {}
+  def objectARRAY_94(x: Array[_ <: Null]): Unit = {}
+
 }